Mail Python API Overview

App Engine applications can send email messages on behalf of the app's email
receiving addresses and on behalf of some users with Google Accounts. Apps can
receive email at various addresses. Apps send messages using the Mail service
and receive messages in the form of HTTP requests initiated by
App Engine and posted to the app.

Sending mail

The Mail service can send email messages to one or more recipients. A message
contains a subject, a plaintext body, and an optional HTML body. It can also
contain file attachments and a limited set of headers.

You can use any email address for a recipient. A recipient can be in the
message's "to" field, in the "cc" field, or hidden from the message recipients
(a "blind carbon copy" or "bcc").

When an application calls the Mail service to send a message, the message is
queued, and the call returns immediately. The Mail service uses standard
procedures for contacting each recipient's mail server, delivering the message,
and retrying if the mail server cannot be contacted.

Who can send mail

For security purposes, the sender address of a message must be one of the
following:

The Gmail or G Suite Account of the user who is currently signed in

Any email address of the form anything@[APP_NAME].appspotmail.com or
anything@[APP_ALIAS].appspotmail.com

All email addresses on the
Email API Authorized Senders list need to
be valid Gmail or Google-hosted domain accounts. App Administrators can add the
following accounts to the list of Authorized Senders:

Their own email address

Any group for which they are an Owner or Manager

Applications hosted in a G Suite domain: noreply@[DOMAIN].com, as long
as noreply@[DOMAIN].com is a valid
account (user or group).

In addition, domain administrators of domains managed by G Suite can add
any user in their domain to the list of authorized senders.

You are limited to a maximum of 50 authorized senders.

Mail from G Suite

If you will be sending email from a domain managed by G Suite, you should
set the DNS SPF records for your domain to indicate that Google is a trusted
source for your email. For instructions on how to do this, see SPF
records
in the G Suite help articles.

Note that if you use aliases set up for your G Suite domain, you cannot
send email from email addresses that use the domain alias.

Bounce notifications when mail is not delivered

Mail that matches a known signature for spam, viruses, or other malicious
content may not be accepted for delivery.

If the Mail service cannot deliver a message, or if a recipient's mail server
returns a bounce message (for example, because there is no account for that
address on that system), an error message can be sent by email to the address of
the sender for the message. The application itself does not receive any
notification about whether delivery succeeded or failed.

By default, email bounce notifications are not enabled. In order to receive
email bounce notifications, you need to configure your application. For more
details, see Receiving Bounce
Notification.

Mail and the development server

The development server can be configured to send email messages directly from
your computer when you test a feature of your app that sends messages. You can
configure the development server to use an SMTP server of your choice.
Alternatively, you can tell the development server to use Sendmail, if Sendmail
is installed on your computer and set up for sending email.

If you do not configure an SMTP server or enable Sendmail, when your app calls
the Mail service, the development server will log the contents of the message.
The message will not actually be sent.

Authentication with DomainKeys Identified Mail (DKIM)

When you send mail, App Engine uses the DomainKeys Identified Mail
(DKIM) standard to authenticate the domain. If your application sends messages
from an email address that is part of a G Suite domain, App Engine can
cryptographically sign the emails it sends. This signature says that an email
that purports to be from emma@example.com really came from example.com. The
recipient can check this signature; if the signature is correct, the recipient
knows that the sender's domain wasn't spoofed.

To enable DKIM authentication for messages sent from G Suite email addresses,
follow these instructions
in the G Suite Help Center. Note that it can take up to 48 hours before DKIM
authentication is active for your G Suite domain.

You must also ensure that your application is using a custom
domain
and is the same as your G Suite domain. Also note that G Suite domains
with subdomains such as subdomain.example.com are not permitted.

App Engine will sign the application's outgoing mails if the sender
address is part of a G Suite domain with DKIM enabled. Additionally, the sender
address must be formatted such that the domain part of the email address only
consists of lowercase letters.

Quotas and limits

To see the current quota usage of your application, go to the Quota Details page
in the Google Cloud Platform Console.

Each Mail service request counts toward the Mail API Calls quota. There are
also quotas for the number of messages you can send, the number of Admin
messages you can send, and the amount of data and attachments you can send. For
more details, see Mail quotas.

Note: Data sent in the body of an email message or as an attachment also counts
towards the Outgoing Bandwidth (billable) quota.

The quota is reset daily. If you exceed the daily quota, you will get an over
quota exception. If you
need to send more mail than the quota allows, you can use a third-party mail
provider, such as SendGrid or
Mailgun.

In addition to quotas, the following limits apply to the Mail service: